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Pond UV Lights: A Beginner’s Guide

3rd May

What are Pond UV lights?

Eco-systems such as ponds require continuous care and maintenance. This ensures that aquatic life (whether it’s animals or plants) are healthy and are able to thrive in their environment. Pond UV lights or UV ‘filters’ are ultraviolet light systems that have been used for years in ponds and aquariums in an effort to control green water caused by algae.

How do UV filters work?

The first thing to note is that UV filters are technically not filters. By definition, UV systems do not function as filters. That’s because they don’t actually remove any unwanted material. UV clarifiers, as they are better understood, function as sterilisers because they work simply by killing algae. however, the algae still remain in the water because UV Clarifiers do not remove them.

UV lights work when water from your pond is pumped through their light path. The single-celled, free-flowing algae within your pond water is then exposed to these high levels of ultraviolet light. This light destroys their DNA and kills them.

The damage caused to the algae’s DNA through this light also causes them to flocculate (clump together). Making the algae easier to trap and remove from the pond water. As mentioned before, the UV system does not remove algae from the pond, it simply kills them.

What to consider before relying on a UV filter?

When it comes to eliminating green water algae in your pond, UV lights should be the last method you turn to. While pond UV lights work to some extent, they do have a few disadvantages.

The biggest concern with UV systems is that they only work to tackle single-celled alga. They aren’t able to combat all forms of algae such as floating and submerged blanket weed. This is because floating blanketweed is only present on the surface of pond water. Thus is never exposed to the UV light. Similarly, submerged blanket weed algae is anchored to things such as rocks, plants and the sides of the pond. There, it also never passes through the UV light system.

Not sure what blanket weed is?

Check Out Our Ultimate Guide to Blanket Weed

Although UV lights are good for eliminating algae is water through sterilisation, this same process kills both bad AND good bacteria. Good bacteria is essential for healthy pond chemistry and prevent it from becoming unbalanced through the build-up of chemicals from natural organics and fish waste.

You should also note that UV lights need to be replaced in order to work. Over time, the bulbs for the lights become weaker and should be replaced yearly to remain effective at clearing algae. Be aware that bulbs may still light up, but this does not mean they are still emitting UV rays, also in relation to this you should consider the quality of your light system. Cheap UV lights will not have the same strength and effect as more expensive options.

How to use UV filters with our products 

The good news is that there is a more advanced alternative way to eliminate the build-up of algae in your pond, not just green water but other forms of algae such as submerged blanketweed/string algae or floating blanket weed. As well as other pond pollutants such as sludge and organic matter.

The answer is beneficial bacteria. Our pond products use beneficial bacteria that work to digest algae and its source of food. This means algae can be eliminated from your pond and you can control/prevent it from recurring.


Provided this and that UV lights kill ALL bacteria, good and bad. We advise leaving the UV lights off for 2 days after applying our bacterial products to your pond in order to allow them to anchor onto something and begin to replicate to do their job.

There is an argument that if you have UV lights in your pond and still get green water algae, that your UV system might not actually work. So, it may not be worth it to turn it back on. Overall, regular doses of bacteria in your pond will be far more effective for ponds than UV lights in providing a biologically balanced pond.

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David fieldman

I use your products and done away with my uv light my pond is now clear no alge koi are doing great